The present invention relates to a forced transport, electrically excited molecular gas laser and a method of improving the discharge uniformity in such a laser. More particularly, the invention is directed to an improved high power forced transport, electrically excited molecular gas laser having a large volume discharge in a gaseous medium at a pressure of at least 50 Torr and to a method of improving the discharge uniformity thereof.
High power molecular gas lasers, such as fast axial flow CO.sub.2 lasers, must be able to maintain uniform discharge properties at vacuum and pressure as compared to conventional gas lasers. However, as the gas pressure increases to high pressures of between 50 and 150 Torr, discharge uniformity and arc formation become limiting parameters. That is, it is generally impossible to strike a large cross sectional area discharge between the two parallel plates in such high pressure gases. Instead, a single arc will be established. The diameter of the arc is restricted by (a) electron emission at the cathode due to bombardment of positive ions, and (b) the electrodynamics of the column; increasing current, increases ionization which decreases the potential gradient and constricts the column as discussed in U.S. Pat. Nos. 3,735,284 and 3,795,838.
To promote discharge uniformity and restrict arc formation it has been proposed in the aforementioned U.S. patents to control the ion spacial distribution, and therefore indirectly the electron spacial distribution in large volume discharges, by means of aerodynamic forces created by partially obstructing the large volume discharges. More specifically, in the identified patents, horizontal and vertically extending rods are placed transverse to the laser gas flow path to create vortices in the gaseous flow to rapidly diffuse the plasma. Further mixing also results from the placement of nozzles in the gas flow which shock supersonic flow back to subsonic. However, the provision of such devices in the gaseous lasing medium flow path is problematical in that they reduce the cross sectional area of the flow passage which increases the pressure drop along the flow passage. This in turn reduces the operating efficiency of the laser because, for a given blower capacity, an increased pressure drop results in a decrease in the rate at which the gaseous lasing medium is moved through the discharge length of the laser. In particular, lower gas flow rates reduce the power output of the laser because they reduce the mass flow rate. Lower gas flow rates also reduce the cooling from the gas flow which likewise limits the laser output capacity since once the gas temperature exceeds 300.degree. C., for example, thermal bottlenecks occur and the gas is too hot to lase.
An object of the present invention is to provide a high power, forced transport, electrically excited molecular gas laser which provides improved ionization and cooling while avoiding the aforementioned problems of the prior art devices.
A further object of the invention is to provide an improved forced transport, electrically excited molecular gas laser having a large volume discharge in a gaseous medium at a pressure of at least 50 Torr which allows high pressure and fast axial flow operation, the laser having a compact design which produces high power optical output with uniform and stable mode characteristics.
These and other objects of the invention are attained by providing a forced transport, electrically excited molecular gas laser having a large volume discharge and a gaseous lasing medium at a pressure of at least 50 Torr comprising means defining a flow path for a gaseous lasing medium, a gaseous lasing medium having a pressure of at least 50 Torr contained in the flow path, means for moving the gaseous lasing medium at high speed along the flow path, means for electrically exciting the moving gaseous medium along an active discharge length of the flow path to create a plasma for lasing, and wherein the means defining the flow path includes expansion chamber means in the vicinity of the active discharge length of the flow path for rapidly expanding the moving gaseous lasing medium to stir the plasma and provide a uniform and stable discharge.
While it is known to provide enlargements such as bell-shaped end portions in laser discharge tubes to accommodate electrodes, these areas are substantially obstructed by the electrodes or other structures contained therein. However, it has been discovered that the provision of substantial additional, essentially unobstructed, space in the form of an expansion or ionization chamber in the vicinity of the active discharge length of the flow path effectively mixes the ions and the plasma as the gas moves into and out of this chamber via the adjacent, relatively smaller cross sectional area portions of the flow path. According to the invention, a single expansion or ionization chamber may be provided at the upstream, anode end of the laser flow path or several expansion chambers may be provided along the length of the active discharge in the laser flow path to increase the permissible active length by reducing the tendency for downstream arcing.
According to a disclosed, preferred embodiment the means defining a flow path for the gaseous lasing medium includes an elongated laser discharge tube with a bore for the passage of the gaseous lasing medium. The expansion chamber means is defined by an enlarged portion of the laser discharge tube bore located intermediate the ends of the tube. The enlarged portion of the laser discharge tube has a cross sectional area at least approximately six times that of the adjacent bore for effecting the rapid expansion of the moving gaseous medium and stirring of the plasma.
According to a second disclosed embodiment of the invention the laser discharge tube includes a plurality of expansion chamber means in the form of respective enlarged portions of the bore of the tube spaced along the length of the tube to periodically rapidly expand the moving gaseous lasing medium to stir the plasma. Equal potential means may be provided in the expansion chamber means for establishing a uniform electric field across the plasma to further increase the output power or capacity of the laser. Additional power from the laser can also be obtained according to the invention by connecting at least two of the equipotential means to radio frequency excitation means to electrically excite the gaseous medium moving in the discharge tube.
In the several disclosed embodiments the primary electrical excitation of the moving gaseous medium along an active discharge length of the flow path is attained by the application of direct current across an anode and a cathode positioned in spaced relationship in the discharge tube along the flow path. The elongated discharge tube includes a gas inlet fitting extending transverse to the longitudinal direction of the discharge tube. The inlet fitting defines an inlet bore portion for the passage of the gaseous lasing medium. The inlet bore portion communicates directly with an expansion chamber means provided in the vicinity of the anode. In one form of the invention, an additional expansion chamber means is provided downstream intermediate the anode and cathode.
The anode is preferably in the form of a rod with one end portion of the rod extending into the expansion chamber means whereby the expansion chamber means is essentially unobstructed to permit the rapid expansion therein of the moving gaseous lasing medium. Anode support means are provided for supporting the anode rod from its end opposite the one end. The anode support means is positioned at an end of the elongated discharge tube. The cathode in the illustrated embodiments is an annular member which encircles the flow path. The cathode is preferably removably mounted in an enlarged end portion of the discharge tube by cathode support means connected to an end of the discharge tube.
A method according to the invention for improving the discharge uniformity in a forced transport, electrically excited molecular gas laser having a large volume discharge in a gaseous medium at a pressure of at least 50 Torr comprises the steps of moving a gaseous lasing medium having a pressure of at least 50 Torr at high speed along the flow path, electrically exciting the moving gaseous medium along an active discharge length of the flow path to create a plasma for lasing, and rapidly expanding the gaseous medium in an expansion chamber provided along the flow path in the vicinity of the active discharge length of the flow path to stir the plasma and provide a uniform and stable discharge.
Further, according to the invention, the step of rapidly expanding the moving gaseous medium may be performed a plurality of times as the gaseous medium moves along the active discharge length of the flow path to thereby increase the effective length of the laser and enhance the laser output. Laser output can be further enhanced by the additional step of equalizing the electrical field across the plasma between the rapid expansions of the moving gaseous medium.
These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the invention.